This invention relates to focussing arrangements for camera tubes in which the focus current and focus voltage track in a square or square-root relationship.
Camera tubes such as vidicons require focussing of the electron beam. This focussing is normally accomplished by an electrostatic focus electrode in the form of a cylinder or tube through which the electron beam passes on its way from the electron gun to the photosensitive surface. A high positive direct potential is applied to the focus tube. In conjunction with the electrostatic focus, a solenoidal focus coil through which a focus current passes is wound about the exterior of the camera tube to aid in focussing.
The magnitude of the focus current depends upon the number of turns in the coil, the amount of focussing desired and the like.
In the past, the focus voltage and focus current applied to the focus tube and focus coil, respectively, have been manually adjusted to particular values providing optimum focus. For less critical applications, such manual adjustments are satisfactory. However, in the presence of component aging, power-supply voltage variations, environmental changes such as temperature changes, and the like, defocussing of the electron beam may occur.
Cameras intended for critical applications, such as studio cameras, use feedback regulators to maintain the focus voltage and focus current to within very close tolerances, such as 1/2%, of the predetermined optimum value.
U.S. Pat. No. 3,355,621 issued Nov. 28, 1967 to W. J. Cosgrove describes an arrangement in which the need for close regulation of the focus voltage and focus current is reduced. In the Cosgrove arrangement, the focus voltage is slaved to or controlled by the focus current in such a manner that incremental changes away from the preset focus current create proportional changes in the focus current. A "proportional change" for this purpose means that the percentage change in the magnitude of the focus voltage equals the percentage change in the focus current to which the focus voltage is slaved.
Modern surveillance and industrial cameras must meet high technical standards in a highly cost-competitive market. It is desirable to provide improved focus performance without the cost of high-accuracy regulation of the focus voltage and current.